Migrating cells often exhibit signal relay, a process in which cells migrating in response to a chemotactic gradient release a secondary chemoattractant to enhance directional migration. In neutrophils, signal relay toward the primary chemoattractant N--formylmethionyl-leucyl-phenylalanine (fMLP) is mediated by leukotriene B4 (LTB4). Recent evidence suggests that the release of LTB4 from cells occurs through packaging in exosomes. Here we present a mathematical model of neutrophil signal relay that focuses on LTB4 and its exosome-mediated secretion. We describe neutrophil chemotaxis in response to a combination of a defined gradient of fMLP and an evolving gradient of LTB4, generated by cells in response to fMLP. Our model enables us to determine the gradient of LTB4 arising either through directed secretion from cells or through time-varying release from exosomes. We predict that the secondary release of LTB4 increases recruitment range and show that the exosomes provide a time delay mechanism that regulates the development of LTB4 gradients. Additionally, we show that under decaying primary gradients, secondary gradients are more stable when secreted through exosomes as compared with direct secretion. Our chemotactic model, calibrated from observed responses of cells to gradients, thereby provides insight into chemotactic signal relay in neutrophils during inflammation.
© 2017 Szatmary et al. This article is distributed by The American Society for Cell Biology under license from the author(s). Two months after publication it is available to the public under an Attribution–Noncommercial–Share Alike 3.0 Unported Creative Commons License (http://creativecommons.org/licenses/by-nc-sa/3.0).